The requirement of a high ignition voltage is a well-known characteristic of gas discharge lamps. The most common types of discharge lamps, such as fluorescent lamps, typically require starting voltages on the order of hundreds of volts. Other types of discharge lamps, such as high-intensity discharge (HID) lamps, require much higher starting voltages on the order of several thousand volts.
Many types of existing ballasts for fluorescent lamps include a high frequency (e.g., 20,000 hertz or greater) voltage-fed inverter and a series resonant output circuit. These ballasts provide a high voltage for igniting the lamp and efficiently limit the steady-state operating current of the lamp. While these types of ballasts work well for the most common kinds of gas discharge lamps, such as fluorescent lamps, they are not well suited for HID lamps. First, HID lamps are prone to acoustic resonance effects that occur in certain frequency bands above one thousand hertz or so. Consequently, high frequency operation, which is a practical requirement for resonant inverter ballasts, is problematic for HID lamps. Second, HID lamps require much higher starting voltages than fluorescent lamps. For example, whereas a T8 type fluorescent lamp ordinarily requires less than 1000 volts (peak) to ignite, an HID lamp typically requires about 3000 volts (peak). Resonant circuits are generally not capable of efficiently and cost-effectively providing such high starting voltages, particularly at low operating frequencies.
The prior art teaches a number of ballasts and starting circuits for HID lamps. Many HID ballasts include dedicated starting circuits that employ active breakdown devices, such as sidacs, to generate a momentary high voltage for igniting the lamp. Such devices are undesirable because of their monetary cost. Moreover, these devices often require a significant amount of control circuitry, which adds additional cost and complexity to the ballast.
What is needed, therefore, is a highly efficient and cost-effective HID ballast that provides low frequency operating power and a high starting voltage, but that does not require costly active devices and associated control circuitry to generate a high starting voltage. Such a ballast would represent a considerable advance over the prior art.